A standard coextrusion film has a core layer made of a thermoplastic elastomer first and second outer layers each formed from a polymer having a lower elasticity than the thermoplastic elastomer. The outer layers each have a layer thickness of less than 15 μm and empty or gas-filled cells formed by foaming.
Such a coextrusion film is particularly provided as an elastic material for disposable hygiene products and is used, for example, as an elastic waistband or as an elastic closure element in a diaper. Even though high demands are placed on the second stretching behavior and the elastic return force in such an application, the material must be as cost-effective as possible in view of the unit quantities of corresponding disposable products.
It must be kept in mind that multilayered laminates are frequently used for the abovementioned applications in which the coextrusion film us on one or even both sides over additional layers, particularly top layers of nonwoven.
A film with an elastic core layer and inelastic outer layers is known from U.S. Pat. No. 5,691,034. The outer layers are relatively stiff and are made, for example, of a polyolefin, particularly polyethylene or polypropylene, a polyethylene terephthalate or polyacrylate. The outer layers have a microtexture that has been produced by stretching the laminate beyond the elastic limit of the outer layers. The microtexture consists of small folds that are formed upon the elastic return of the film after second stretching, the alignment of the folds being dependent on the direction of second stretching. The film can be stretched uniaxially or also biaxially. The microtexture is perceived as a roughened, soft surface texture.
A coextrusion film is known from U.S. Pat. No. 7,449,240 that has a core layer made of a thermoplastic elastomer and outer layers made of a less elastic material. The outer layers consist of a thermoplastic plastic with a brittle consolidated molecular structure that stretches only a little when tensioned and breaks without transition when a predefined breaking limit is reached. The brittle, consolidated molecular structure is irreversibly broken open by a uniaxial second stretching transverse to the direction of extrusion that is referred to as an activation. As a result, the coextrusion film can be stretched elastically transverse to the direction of extrusion. The outer layers can be made of a polyolefin made brittle through post-treatment, a mixture of polyolefin and polystyrene or a mixture of polyolefin, polystyrene and ethylene vinyl acetate. In the direction of extrusion, which is also referred to as the machine direction, the brittle consolidated molecular structure remains intact and stiffens the material. The layered composite has no elasticity in the machine direction.
A coextrusion film with the features described above is known from US 2012/0164383. The coextrusion film has pores formed in the top layers through foaming and subsequent second stretching. By stretching the coextrusion film, which is also referred to as activation, the coextrusion film develops elastic characteristics and a textured top layer at the same time. The pores in the top layer improve the elastic characteristics of the coextrusion film that is reversibly stretchable to a great extent after its activation. The pores also result in a matte, irregular surface.